1. Field of the Invention
The present invention relates to a hard disk drive performing a seek control method using a multi-sinusoidal wave acceleration profile, and more particularly, to a seek control method capable of coping with a variation in a voice coil motor driving current accompanying variances in a peripheral environment, a recording medium storing a program to execute the method, and a hard disk drive adopting the method.
2. Description of the Related Art
In general, a hard disc drive (HDD) includes a plurality of magnetic heads for writing and reading information by magnetizing a rotating disc and sensing a magnetic field from the disc. The information is stored on concentric tracks. Each track has a unique disc number and a track number. In a plurality of discs, tracks having the same track number are called a cylinder. Therefore each track can be defined by its cylinder number.
Each head (or transducer) is typically integrated into a slider assembled with a head gimbal assembly (HGA). Each HGA is attached to an actuator arm. The actuator arm has a voice coil that is located adjacent to a magnetic assembly which defines a voice coil motor (VCM) together. Further, the HDD includes a VCM driving circuit, which supplies a current for exciting the VCM, and a controller. The excited VCM rotates the actuator arm to move the head across the disc.
When writing or reading information, the HDD may perform a seek control routine for moving the head from one track to another. During the seek control routine, the VCM is excited to move the head from a certain track to a target track. The controller controls the current for exciting the VCM to move the head exactly to the target track.
It is preferable to minimize the time required to read or write information from or to discs. Therefore, the seek control routine performed by the HDD needs to move the head to the target track as quickly as possible. In addition, a settling time of the HGA should be minimized so that the head can write or read the information as quickly as possible.
In a conventional method, the seek control is performed to move the head to the target track using a square wave acceleration profile. Unfortunately, a square wave includes harmonic waves of high frequencies. These harmonic waves result in a mechanical resonance of an HGA, resulting in mechanical components or assemblies vibrating. In addition, residual vibration causes audible noise. Further, the mechanical resonance generated by the seek control method using the conventional square wave acceleration profile causes an increase in both the settling time required to write or read information on or from discs and the overall seek time.
To resolve this problem, a seek control method using a sinusoidal wave acceleration profile has been developed. A seek control method using the sinusoidal wave acceleration profile has advantages in terms of vibration and audible noise compared to the seek control method using the square wave acceleration profile. Nevertheless, the seek control method using a sinusoidal wave acceleration profile has disadvantages in that a seek time is increased by as much as 10% compared to the seek control method using the square wave acceleration profile. This is easily understood by comparing a sinusoidal wave and a square wave. An area under a sinusoidal wave is smaller than that occupied by a square wave. This area indicates the amount of current provided to accelerate or decelerate the VCM. Because the amount of current in the seek control method using a sinusoidal wave acceleration profile is smaller than that used in the seek control method using the square wave acceleration profile, the VCM has less momentum, and thus the seek time is longer when seeking the same distance.
A multi-sinusoidal wave seek control method has been developed to overcome this disadvantage. The multi-sinusoidal wave seek control method is disclosed in Korean Patent Publication Nos. 2001-62386, which was filed on Jul. 7, 2001, and 2001-67380, which was filed on Jul. 12, 2001. While the sinusoidal wave seek control method uses a single sinusoidal wave, the multi-sinusoidal wave seek control method uses a harmonic wave obtained by synthesizing at least two sinusoidal waves.
FIG. 1 is a diagram illustrating a multi-sinusoidal wave acceleration profile used in the multi-sinusoidal wave seek control method. The multi-sinusoidal wave acceleration profile is obtained by synthesizing a plurality of sinusoidal waves each having different frequencies. An acceleration period is depicted as being symmetrical to a deceleration period in FIG. 1. However, in most cases, the acceleration period and the deceleration period are asymmetrical. A main reason for the asymmetry is that the multi-sinusoidal wave acceleration profile is obtained by synthesizing at least two sinusoidal waves. The asymmetry also occurs because a ratio of the acceleration period to the deceleration period is varied in order to reduce the residual vibration of the mechanical components and to shorten the settling time. This is done by varying the synthesizing ratio of sinusoidal waves.
Typically, the seek control can be performed in an acceleration mode, a deceleration mode, and a coast mode in which the head coasts at a maximum speed for long distance seeking. In general, a maximum value of a current input to a VCM in the seek control is limited based on the performance and mechanical vibration of the VCM. That is, in the acceleration mode, the maximum speed to which the head is accelerated is limited according to the maximum current. Also, since an acceleration profile and a deceleration profile should be symmetrical if at all possible, the deceleration mode is performed when the head reaches the maximum velocity. Accordingly, a distance, which can be sought only with the acceleration mode and the deceleration mode, is limited. Therefore, the coast mode is necessary to seek a distance longer than a predetermined distance.
FIG. 2 is a diagram illustrating an acceleration profile for the multi-sinusoidal wave seek control method in which the coast mode is used. In the coast mode, a value of the current input to the VCM is 0. In detail, the head is accelerated by applying current to the VCM in the acceleration mode and when the velocity of the head is at a maximum, i.e., at a position A of FIG. 2, the acceleration mode ends and the coast mode begins by blocking the current input to the VCM. The head is not accelerated anymore and coasts at a maximum speed due to inertia. After coasting a predetermined distance, the coast mode ends and the deceleration mode begins at a position B. In the deceleration mode, the velocity of the head is decelerated by applying opposite current to the VCM. Accordingly, the head stops on the target track. For accurate seek control, the conversion to the coast mode and the conversion to the deceleration mode should be precisely controlled. Accurate control is more important when considering that the head moves at the maximum speed in the coast mode.
FIG. 3 is an equivalent circuit of the VCM. Referring to FIG. 3, the VCM can be represented by a coil resistor Rm and a coil inductor Lm. The resistance of the coil resistor Rm is strongly dependent on temperature.
Variation in the coil resistance of the coil resistor Rm according to a variation in temperature results in variation in current through the VCM, and thus the variation in temperature should be considered in the seek control.
If temperature increases or power supplied to a VCM driving unit varies, then the HDD using a single sinusoidal wave seek method cannot generate a desired maximum VCM driving current. Accordingly, a waveform of a current for seek control deteriorates, resulting in a malfunction or a delay in a seek operation. To solve such a problem, a conventional method of adaptively varying the waveform of current in response to variations in peripheral environments, for instance, variations in both temperature and voltage, has been used. Malfunction and delay in seek operations are prevented by calculating a variation ratio of the VCM driving current, i.e., a seek time scaling (STC), according to variations in temperature and VCM driving power, and varying the waveform of the VCM driving current accordingly.
Nevertheless, it is more difficult for the HDD using the multi-sinusoidal wave seek control method to adaptively cope with the variations in temperature and VCM driving power using the STC than the single sinusoidal wave seek control method. This is because, unlike in the single sinusoidal wave seek control method, an acceleration period is not symmetrical to a deceleration period in the multi-sinusoidal wave seek control method, and thus it is difficult to adapt the STC uniformly.
The complexity in seek control increases further in the multi-sinusoidal wave seek control using a coast mode because a distance for coasting varies according to the variation in the VCM driving current.
In a long distance seek operation, it is sufficient to consider only the variation in the VCM driving current presented by the STC in the single sinusoidal wave seek control because an acceleration period, a deceleration period, an acceleration distance, a deceleration distance, etc. have a linear relationship with the VCM driving current.
However, other parameters besides the variation in a maximum VCM driving current represented by the STC should be considered in the multi-sinusoidal wave seek control because a coast speed is only determined linearly with respect to the VCM driving current. The acceleration period, the deceleration period, the acceleration distance, the deceleration distance etc., all depend on additional parameters.